1. Field of the Invention
The present invention relates generally to a post-tensioning tendon sheathing stripper for use in concrete structures utilizing unbonded post-tensioning systems and, more specifically, to a device for removing the plastic sheathing from the stressing end of tendons.
2. Description of the Background
Post-tensioning concrete entails the use of high-strength steel strand, “tendons,” that are embedded in concrete and tensioned after the concrete hardens. Using tendons under tension creates cast-in-place and precast concrete members that have superior strength characteristics when compared to similarly sized non-prestressed members. In unbonded post-tensioning applications, the steel tendons are first encased in a plastic sheathing before being laid into concrete forms. Most tendons have a fixed anchor on one end that is attached to the tendon in a manufacturing facility and that butts up against the concrete form. The other end of the tendon, also known as the “stressing tail,” is passed loosely through a stressing anchor that is affixed to the other end of the concrete form and then extends a fixed distance past the form. After the concrete is placed, cured, and hardened to a specified strength, a hydraulic jack is attached to the stressing tail to apply tension to the tendon.
The stressing anchor has a tapered pocket into which wedges are placed. The wedges grip the tendon and lock it in place to maintain the tension during the lifespan of the concrete member.
The tendons are encased in a plastic sheathing to protect it from corrosion and to allow the tendon's steel strand portion to move within the concrete member while being tensioned to evenly distribute the tension along the entire tendon. For maximum durability, the sheathing should cover the complete length of the tendon and not be too short; however, the sheathing cannot extend inside of the stressing anchor's wedge pocket because it would impede the clamping ability of the anchor wedges.
There are two ways to ensure that the sheathing is the proper length. The first way is to place the sheathed tendon in its eventual position by laying the stressing tail over the stressing anchor and form. The sheathing is then “ring-cut” at the back of the stressing anchor and slid six to twelve inches away from the ring cut and then slid through the stressing anchor. Construction code requires that the tendon sheathing protrude into the transition sleeve on the back of the stressing anchor at least four inches. At this point, the concrete can be placed and cured, the form can be removed, and the tendon can be tensioned. Repair to the sheathing is necessary if a four inch overlap is not achieved during installation. This repair process comprises making a ring-cut 18 to 24 inches from the end of the anchor sleeve and sliding the sheathing into the anchor sleeve, leaving a one-inch gap. A smaller piece of sheathing can then be slit and installed in the tendon's gap with two layers of waterproof tape.
However, if the sheathing is not ring-cut prior to placing the concrete, the sheathing on the stressing tail must be stripped away from the tendon all the way down to the bottom of the stressing anchor wedge pocket. The present device addresses this situation. There have been a few attempts at developing a device that strips away excess tendon sheathing after the concrete has been placed; however, these existing devices have shortcomings.
U.S. Pat. No. 6,108,910, issued on Aug. 29, 2000, to Sorkin discloses a device and method for stripping the plastic sheathing from a tendon. This device comprises a cylindrical tool body attached to a narrow cutter head on one end designed to fit within the tapered pocket of a stressing anchor. A U-shaped channel runs longitudinally through the tool body and cutter head, and a blade is mounted via a set screw at an acute transverse angle of between 70 and 89 degrees on the cutter head. The channel allows the tool to be placed over the tendon rather than threaded along the tendon. The device is slid into the tapered pocket of the stressing anchor until the blade engages with the sheathing and is then rotated. When the tool is rotated, the blade angle causes a “drawing inward” effect until the tool abuts the anchor, and a ring cut is produced at the stressing anchor end of the excess sheathing. At this point the blade cuts transversely through the sheathing, and the waste sheathing can be removed. Unfortunately, the '910 device does not fully spiral cut or remove the excess sheathing, and as a result, the excess sheathing must be removed either by un-threading the sheathing off the tendon or by using a separate knife to slice the waste sheathing longitudinally before removing it from the tendon. Additionally, the use of a stamped steel blade with this device would require frequent blade changes, and the U-shaped design complicates the hand rotation.
U.S. Pat. No. 6,098,290, issued on Aug. 8, 2000, to Sorkin, discloses a device and method for stripping a tendon similar to the device disclosed by the '910 patent, except that the '290 patent incorporates specific features such as a ribbed, compressible tool body that when squeezed grabs the waste sheathing and a removable locking blade of a particular design. Unfortunately, as with the '910 device, the '290 device does not fully spiral cut or remove the excess sheathing, and as a result, the excess sheathing must be removed either by unthreading the sheathing off the tendon or by using a separate knife to slice the sheathing longitudinally before removing it. Additionally, the blade used in this device dulls quickly and must be changed often, and the compressible tool body, combined with the U-shaped design, may make the device difficult to rotate by hand.
U.S. Pat. No. 5,632,088, issued on May 27, 1997, to Naso et al., discloses a tendon stripper device. This device comprises a cylindrical tool body with one tapered end designed to fit within the tapered pocket of a stressing anchor. A cylindrical passage runs longitudinally through the tool body, and an inwardly projecting blade is mounted in a channel in the tapered end at a transverse angle. The blade is mounted on a movable lever such that insertion of the device into a stressing anchor causes the blade to engage the sheathing. Upon engagement, force (such as with a hammer) is then applied to the non-tapered end of the device, causing the blade to “plow” through the sheathing until the blade abuts the bottom of the stressing anchor. Upon full engagement, the device is rotated to cut the sheathing. The waste sheathing can then be removed. Unfortunately, the '088 device requires hammering to become fully engaged, and if the device is not fully engaged, sheathing will remain in the area to be occupied by the anchor wedges causing a sub-optimal locking of the tendon. Additionally, the waste sheathing must be removed either by unthreading the sheathing off the end of the tendon or by using a separate knife to longitudinally slice the sheathing before removing it.
U.S. Pat. No. 5,745,996, issued on May 5, 1998, to Kenny et al., discloses a sheathing cutting device. This device comprises a cylindrical tool body with one tapered end designed to fit within the tapered pocket of a stressing anchor. A U-shaped channel runs longitudinally through the tapered end, and a box-shaped channel runs longitudinally through the rest of the body. An inwardly projecting blade is mounted in a channel in the tapered end at a transverse angle utilizing a spring mechanism. The blade is mounted on a movable lever such that at rest, the blade is retracted within the tapered end of the body, and insertion of the tapered end of the device into a stressing anchor causes the blade to project into the sheathing with the blade being at full projection upon full insertion of the device into a stressing anchor. Once the blade initially contacts the sheathing, force (such as with a hammer) is then applied to the non-tapered end of the device, causing the knife to “plow” through the sheathing as it continues to project until the blade abuts the bottom of the stressing anchor and is fully projected. Upon full projection of the blade and engagement of the device, the device is rotated to cut the sheathing. The waste sheathing can then be removed. Unfortunately, the '996 device requires hammering to become fully engaged, and if the device is not fully engaged, the sheathing will not be fully cut. This result makes removing the sheathing difficult and causes sub-optimal locking of the tendon in the area to be occupied by the anchor wedges. Additionally, the waste sheathing must be removed either by threading the sheathing off the end of the tendon, or by using a separate knife to longitudinally slice the waste sheathing before removal. Finally, the U-channel design may make the device difficult to rotate by hand.